Photo-cathodes for electronic discharge tubes

ABSTRACT

A METHOD OF MANUFACTURING AN ELECTRIC DISCHARGE TUBE HAVING A PHOTOCATHODE IN WHICH A SOURCE OF THE PHOTOCATHODE MATERIAL COATED WITH A SEALANT INERT TO THE PHOTOCATHODE MATERIAL IS PLACED WITHIN AN EVACUATED ENVELOPE WHICH IS THEN HEATED FIRST TO A BAKE-OUT TEMPERATURE AND THEN TO A HIGHER CATHODE-FORMING TEMPERATURE.

T. MURPHY May 30, 1972 I'I'IOTO-CATHODES FOR ELECTRONIC DISCHARGE TUBES Filed March 30, 1970 FIST FIGS.

United States Patent US. Cl. 117-201 4 Claims ABSTRACT OF THE DISCLOSURE A method of manufacturing an electric discharge tube having a photocathode in which a source of the photocathode material coated with a sealant inert to the photocathode material is placed within an evacuated envelope which is then heated first to a bake-out temperature and then to a higher cathode-forming temperature.

This invention relates to photo-emissive cathodes for electric discharge tubes and to the manufacture of tubes containing such photocathodes. Examples of such tubes are imaging tubes and photo-multiplier tubes, and antimony is a typical example of photo-cathode material for such tubes.

It is well known that in the construction and manufacture of photo-emissive devices using photocathodes based on antimony, there is a conflict between the temperature to which the device should be baked and the tendency for antimony to evaporate at that temperature. The unwanted evaporation of antimony can cause various defects such as spurious emission, low insulation and in some cases loss of the antimony required for the photocathode.

This conflict can be resolved to a large extent by suppressing the evaporation of antimony. A well known method of so doing is to alloy the antimony with metals such as gold, platinum or palladium. However such alloys have been found to adversely afiect the cathode sensitivity which can be obtained.

The present invention provides a method of manufacturing an electric discharge tube containing a photocathode which method includes a high temperature vacuum baking step, a preceding step in which the source of photo-cathode material is coated with a sealant which is inert to the photo-cathode material to suppress or minimise evaporation of said photo-cathode material during said baking step, and a subsequent photo-cathode forming step in which said source of photo-cathode material is raised to a temperature higher than that of the preceding baking step.

In a typical case the photo-cathode material is constituted at least principally by antimony.

The sealant may be a silicate, for example potassium silicate, soda metasilicate or ethyl silicate.

As a preferred example, potassium silicate has been used on an antimony head. A thin solution applied to the bead and allowed to dry effectively prevents antimony evaporation up to at least 400 C. (and even higher temperatures in some circumstances) and does not prevent the subsequent evaporation of antimony. Moreover the photocathodes formed from such beads have been found to be unaffected by the coating, and indeed such good photocathodes have been made as to suggest that the coating may be beneficial in preventing contamination of the antimony during the bake-out process.

In fact, it appears that the coating may prevent contamination (e.g. by oxidation) while the head is stored before baking, such storage being done in air at a temperature of, say, C. to keep the components dry (actually the coating can thus protect the bead in air even at higher temperatures up to about 300 C.).

An embodiment of the invention will now be described by way of example with reference to the drawing which shows an antimony bead with its two connections, FIG. 1 being an axial section, FIG. 2 an end view and FIG. 3 a side view.

The two connections A-B are of molybdenum wire, e.g. 0.1 mm. thick, having a cranked section on which the antimony bead C is formed. The wire is coated with platinum which has afiinity for the antimony.

The head C is about 1 mm. in diameter and has a coating D of potassium silicate I to 5 thick which covers also the spread of antimony (at E) on either side of the bead proper.

When manufacturing a tube with the aid of such a head, the head is located in an appropriate position inside the tube envelope with the wires A-B connected to external terminals which are provided on the envelope. The tube is then evacuated and baked (e.g. up to, say, 400 C.) without the silicate coating breaking down or allowing escape of antimony. Subsequently the photocathode is formed by passing a heavy current pulse through the A-B connections so as to raise the bead to a temperature much higher than 400 C. thereby evaporating the antimony onto the desired surface or surfaces, such evaporating being directed thereto e.g. by a shield or shields in known manner.

What is claimed is:

1. A method of manufacturing an electric discharge tube having a photocathode comprising the steps of placing a source of photo-cathode material coated with a sealant inert to the photo-cathode material within an evacuated envelope, heating the envelope with the source of photo-cathode material therein to a temperature at which the envelope is baked without breaking down to the sealant coating the source of photo-cathode material, and thereafter raising the temperature to form the photocathode.

2. A method as claimed in claim 1 wherein the photocathode material is constituted at least principally by antimony.

3. A method as claimed in claim 1 wherein the sealant is a silicate.

4. A method as claimed in claim 3 wherein the sealant is potassium silicate.

References Cited UNITED STATES PATENTS 3,178,308 4/1965 Oxley et al 117106 3,127,226 3/1964 Rector 117-l06 X RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 117-106 R 

